A-type lamins are intermediate filament proteins that provide a scaffold for tethering chromatin and protein complexes regulating nuclear structure and function. Interest in lamins increased after the association of mutations in the LMNA gene with a variety of degenerative disorders broadly termed laminopathies, cancer and aging. The causal relationship between alterations of A-type lamins and disease and the molecular pathways involved are poorly understood. Our data revealed novel functions for A-type lamins in the maintenance of telomeres and in the stabilization of 53BP1, a mediator in the DNA damage response pathway. Loss of A-type lamins leads to telomere shortening, telomeric chromatin defects, impaired DNA repair, increased genomic instability, and defects in the non-homologous end-joining of dysfunctional telomeres, a process that requires 53BP1. Some of these phenotypes would be consistent with 53BP1 deficiency. In support of this notion, reconstitution of 53BP1 into A-type lamins-deficient cells rescued defects in DNA repair. Elucidating molecular mechanism responsible for degradation of 53BP1 might provide novel targets for therapy. Our preliminary data indicates that 53BP1 is degraded by the protease Cathepsin L and by the proteasome. Interestingly, a mouse model of progeria (Zmpste24-/-) shown to markedly upregulate Cathepsin L, exhibits low levels of 53BP1, suggesting that deregulation of the Cathepsin L-mediated degradation of 53BP1 might be a common event in laminopathies. Importantly, we found that treatment with vitamin D, a known activator of Cystatin D [8], the endogenous inhibitor of Cathepsin L, rescues 53BP1 levels, providing a putative strategy to prevent 53BP1 degradation, ameliorating phenotypes of laminopathies. The hypothesis to be tested is that alterations in A-type lamins causing upregulation of Cathepsin L lead to destabilization of 53BP1 protein and defects in DNA repair and telomere stability, contributing to genomic instability and the phenotypes of some lamin-related diseases. Our hypothesis will be tested under three specific aims. Under Aim 1, we will elucidate the molecular mechanisms leading to destabilization of 53BP1 in laminopathies and evaluate ways to target this pathway and improve DNA repair. Under Aim 2, we will determine the extent to which deregulation of the Cathepsin L/53BP1 pathway in laminopathies affects telomere stability. Under Aim 3, we will determine whether regulation of the Cathepsin L/53BP1 pathway by vitamin D ameliorates the pathophysiology of lamin-related diseases in vivo. The studies performed here will be of significance not only for elucidating the molecular mechanisms by which A-type lamins impact on telomere function and DNA repair, but also for understanding the underlying basis of the pathophysiology of lamin-related diseases, including degenerative laminopathies, premature aging syndromes and cancer. The results of our study could translate into the development of novel therapeutic strategies that target 53BP1 degradation pathway to improve the phenotypes associated to these diseases.